1. Field of the Disclosure
The present invention relates generally to power converters, and more specifically, the invention relates to voltage regulation of power converters.
2. Background
Many electrical devices such as cell phones, personal digital assistants (PDA's), laptops, etc. are powered by a source of relatively low-voltage DC power. Because power is generally delivered through a wall outlet as high-voltage AC power, a device, typically referred to as a power converter, is required to transform the high-voltage AC power to low-voltage DC power. The low-voltage DC power may be provided by the power converter directly to the device or it may be used to charge a rechargeable battery that, in turn, provides energy to the device, but which requires charging once stored energy is drained. Typically, the battery is charged with a battery charger that includes a power converter that meets constant current and constant voltage requirements required by the battery. Other electrical devices, such as DVD players, computer monitors, TVs and the like, also require a power converter for device operation. The power converter in these devices also has to provide output voltages and currents that meet the requirements of the device. In operation, a power converter may use a controller to regulate output power delivered to an electrical device, such as a battery, that may be generally referred to as a load. More specifically, the controller may be coupled to a sensor that provides feedback information of the output of the power converter in order to regulate power delivered to the load. The controller regulates power to the load by controlling a power switch to turn on and off in response to the feedback information from the sensor to transfer energy pulses to the output from a source of input power such as a power line. One particular type of power converter that may be used is a flyback power converter. In a flyback power converter, an energy transfer element may galvanically isolate the input side of the power converter from the output side. Galvanic isolation prevents DC current from flowing between the input side and the output side of the power converter, and is usually required to meet safety regulations.
Power converter control circuits may be used for a multitude of purposes and applications. There is a demand for integrating control circuit functionality that can reduce the number of components outside the integrated control circuit. This reduction in external component count enables miniaturization of the power converter to improve portability, reduces the number of design cycles required to finalize a power converter design and also improves reliability of the end product. Furthermore, reduced component count can offer energy efficiency improvements in the operation of the power converter and can reduce the power converter cost. Typically, a power converter has special circuits on the output side of the power converter to sense and to transmit feedback information about the output voltage to the control circuit that is on the input side of the power converter. One technique to reduce the number of components in the power converter is to sense the feedback information of the output voltage from the input side of the power converter instead of sensing it on the output side of the power converter. This is accomplished by a means of an indirect feedback. One challenge associated with power converters using indirect feedback is compensating for the varying voltage dropped across a cable that connects the power converter (e.g. battery charger) to the load (battery). Indirect feedback can regulate the voltage at the output of the power converter that is at one end of the cable, but the voltage the other end of the cable will be different from the voltage at the output of the power converter by the voltage drop of the cable. By compensating for the additional voltage drop of the cable, the power converter provides improved voltage regulation at the load.
There are known discrete circuits that are implemented externally to an integrated power supply controller, which can compensate for the voltage drop of the cable. However, the known discrete circuits that compensate for the voltage drop across the cable introduce additional components that increase the cost and size of the power converter. For example, known discrete cable drop compensation circuits may include relatively large capacitors that increase the size of the power converter. In addition, known discrete circuits that compensate for voltage drop across the cable may not be suitable for certain power converters using controllers that implement certain advanced control methods.